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Contributed Talk

Session: : Parasitism And Host-Parasite Interactions 4

COS 309-2 - Pathogen-driven increased mortality can be beneficial – a complex relationship of Daphnia and two symbionts

Thursday, August 10, 2023
3:45 PM – 4:00 PM PDT
Location: D137
Abstract: The outcome of symbiosis is not fixed but, rather, can shift along a mutualism – parasitism continuum in response to ecological changes. In the case of the microsporidium Ordospora pajunii, an observational field study suggested the costs it imposes on the host Daphnia dentifera can be outweighed by benefits when a more deadly fungal parasite, Metschnikowia bicuspidata, is present in the environment; this shifts the relationship between D. dentifera and O. pajunii from parasitism to mutualism. In field-collected hosts, individuals who were infected with O. pajunii benefited by having a reduced probability of having M. bicuspidata spores penetrate the gut. This field evidence suggests interactions between the symbionts, with potential impacts on both host and parasite fitness. Moreover, it suggests that the fitness impact of one symbiont on a host might depend on the community of symbionts present. Here, we tested experimentally whether this interaction impacts fitness of the host and of M. bicuspidata.

To do so, we exposed half of the experimental individuals of our lab clone of D. dentifera to an isolate of O. pajunii early in the host’s lifespan, keeping the other half unexposed; after leaving time for infections to fully establish, we introduced spores of M. bicuspidata to all the experimental units. We quantified host fitness by measuring reproduction and lifespan of host individuals. We also measured production of M. bicuspidata transmission stages in the hosts infected with O. pajunii vs. hosts without microsporidian infection.

Consistent with the pattern in field-collected Daphnia, we found fewer M. bicuspidata spores penetrating the gut in hosts infected with O. pajunii. In addition to being less likely to be penetrated by the fungal spores, hosts infected with O. pajunii yielded fewer fully developed M. bicuspidata spores at the end of infection. This likely resulted from reduced time for development due to earlier host death: the coinfected daphnia died sooner and produced fewer offspring, than Daphnia infected with just M. bicuspidata. Thus, despite the reduction in fungal pathogen fitness, O. pajunii did not have a beneficial effect on individual host fitness. However, given that epidemics of O. pajunii create a dead end for the highly virulent M. bicuspidata within coinfected hosts, host populations are likely to ultimately benefit from the presence of O. pajunii.